The invention relates to a high resolution non-contact near field acoustic imaging system for nondestructive evaluation and testing of materials and components.
During the last decades, several nondestructive techniques based on acoustic wave propagation have been developed for evaluation and testing of materials and components. The ability of acoustic waves to penetrate into the interior of the material has been utilized to obtain critical information about the life-limiting defects in materials as well as components. Acoustic wave propagation has found applications in metals, ceramics, composites, biological and other materials for Non-Destructive Examination (NDE). Often in acoustic testing, the acoustic waves are excited in the material by placing an ultrasonic transducer in direct contact. A coupling medium like water, oil, grease or thin solid material is used between the transducer and the material to provide the direct contact. As the acoustic waves propagate through the material, defects along the path reflect/scatter the acoustic waves. The reflected/scattered signals are detected using the same ultrasonic transducer [pulse-echo] or by placing another transducer on an opposite side, or on the same side in contact with the material [thru transmission]. While this technique allows examination of individual locations, it is extremely time consuming to obtain an image of larger structures. Moreover, the spatial resolution is determined by the diameter of the transducer. In order to enhance the resolution and ease the restriction on scanning, focused acoustic beam techniques in the presence of water were developed. These techniques are known C-scan, C-SAM, and Scanning Acoustic Microscopy. In these methods, the acoustic waves are focused on or in the interior of the test material in the presence of water. The reflected/transmitted signals are detected by a focused transducer, gated, amplified, digitized and stored in a computer. The transducer or the sample may be raster scanned and an acoustic image is constructed using the stored amplitude data. The spatial resolution depends on the acoustic wavelength in water. Typically at 100 MHz the resolution is about 15 μm through water. Utilizing frequencies of the order of 2 GHz, a resolution of about 0.5 μm may be obtained.
Apart from using longitudinal and shear waves, surface acoustic waves (Rayleigh, Lamb, etc.) have been used. The Rayleigh waves propagate on the surface of the material. They penetrate to about one wavelength into the material. The Rayleigh waves are very sensitive to near surface defects. On the other hand, Lamb waves propagate in the entire thickness of the plate. They are also known as plate waves. The Lamb waves are very useful in examining larger structures.
In the acoustic testing methods of thru-transmission and pulse-echo methods, the excitation and detection are separated by several wavelengths of sound distance. The received signal is typically an average over this distance. When the distance between the scattering defect and the receiver is much larger than several wavelengths, the system is known to operate in far field. In general, in far field operation the spatial resolution is limited by the wavelength of the sound as described by Rayleigh criteria. This is also applicable to the C-scan, C-SAM, and acoustic microscopic methods of evaluation where the resolution has been increased by merely focusing the acoustic waves. Instead of operating in the far field, if the instrument can be operated in the near field the resolution is not limited by the wavelength of sound and instead it is determined by the diameter of the sensor.
Focused acoustic beams operating at higher frequencies provide higher spatial resolution. At high frequencies the penetration depth dramatically decreases, minimizing the advantage of in-depth evaluation of the material. Although, acoustic techniques are routinely used for nondestructive evaluation and testing, the material has to be immersed in a fluid, often water. To overcome the water contact problem, specialized air coupled ultrasonic transducers (both unfocused and focused) have been developed as discussed in M. C. Bharadwaj, et al., “Non Contact Ultrasound: Final Frontier in nondestructive materials characterization,” Ceramic Engineering and Science Proceedings, 24th Annual Conference on Composites, Advanced ceramics, Materials and structures: Ed. T. Jensen and E. Ustundag, The American Ceramic Society, Westerville, Ohio (2000). All documents herein referenced are incorporated by reference.
Air coupled ultrasonic transducers operate in air at frequencies in the range of a few hundred kHz to about 5 MHz. The acoustic waves generated by these transducers propagate through air are incident on the sample surface. A large amount of the acoustic energy is reflected, while a small amount will pass through the material. The transmitted signal is detected by another air coupled transducer. There is significant loss of acoustic energy at the sample air interface, because of huge acoustic impedance mismatch between the solid material and air. Thus, the received signal requires an enormous amount of amplification. It also requires significant signal processing to detect the acoustic signal that travels through the material. Although this is a significant improvement, the spatial resolution is limited by the diameter of the transducer and operating frequencies. In fact, the receiving transducer has the same size as the transmitting transducer. It is well known that the amplitude of the received signal is directly proportional to the diameter of the transducer. Thus larger diameter transducers are often used in the experiments. The air coupled ultrasonic imaging has found several applications both in material evaluation and testing as well as in biomedical applications, because it can be performed in air.
In the present invention, we combine the advantages of air coupled transduction and a localized optical detection with a laser interferometer. The acoustic waves in air are excited using an air coupled ultrasonic horn or air coupled ultrasonic transducer. The acoustic waves propagate through the sample and include reflected signals which are detected using non-contacting methods. These non-contacting methods may include but are not limited to a fiber optic displacement sensor or a laser interferometer. The acoustic displacement signal from the optical detector is amplified and stored digitally in the computer. The data at different locations is obtained by a raster scanning a sample in the x and y directions. An acoustic image of the material is built with the stored data.
Another method of complete non-contact acoustic imaging is with the help of laser generated acoustic waves and laser based detection systems. In these methods a high intensity laser periodically impacts the surface of the sample. Due to thermoelastic conversion, an acoustic wave is generated in the material. The propagating acoustic waves cause surface displacements and are detected by a laser interferometer at any other location. By scanning the excitation laser and the detecting interferometer, an acoustic image is developed. The laser ultrasonic methods have high resolution and are completely non contact in nature. One of the major problems with the methodology is the need for high amplitude pulsed lasers for generation of acoustic waves. The possibility of surface damage caused by the high power lasers due to ablation or other heat induced effects may be a limitation of this technique.